Aspects and embodiments of the invention generally pertain to optical apparatus enabling incoherently-induced Coherent Perfect Absorption (CPA), and associated methods and applications thereof. More particularly, aspects and embodiments apply to all linear, lossy, planar photonic structures regardless of the details of their construction. Most particularly, an exemplary, non-limiting device architecture includes a dielectric film placed between two lossless mirrors to form a symmetric or asymmetric resonator realizing complete absorption using an incoherent optical beam over an octave of bandwidth.
Coherent perfect absorption (CPA) is a relatively new optical scheme that produces high absorption in systems that have low intrinsic losses. CPA is the phenomenon where a linear system with low intrinsic loss strongly absorbs two incident beams, but only weakly absorbs either beam when incident separately. By interfering two beams in a lossy material (typically contained in a multi-pass interferometer such as a Fabry-Perot (FP) resonator), increased absorption is observed with respect to that experienced by each beam separately. The effect appears counter-intuitive: while a single beam is weakly absorbed, adding a second beam results in both beams being completely absorbed. This linear phenomenon has been termed ‘lasing-in-reverse’ and studied in terms of the mathematical behavior of the poles and zeros of the system scattering matrix.
Silicon (Si) occupies a privileged position in modern micro-technologies and is now playing a growing role in photonics. Therefore, the ability to control the physical properties of Si, e.g., its optical absorption, could have a profound impact on a variety of applications; for instance, increasing the photodetection efficiency in Si would enable the use of a thin layer, which would lead to an increase in detection speed, while extending its optical absorption into the near-infrared (NIR) would allow harnessing an underutilized portion of the solar spectrum.
A variety of strategies to achieve these goals have been reported. To increase absorption, Si has been placed in cavities to resonantly enhance photodetection. To red-shift the absorption cut-off wavelength, the electronic bandgap of Si has been reduced by applying high pressures that modify the lattice structure. More recently, insights offered by the burgeoning study of non-Hermitian photonic structures are enabling new capabilities by controlling the spatial distribution of the imaginary refractive index component, whether loss or gain. One such insight utilizes interference to increase absorption in low-loss materials so-called ‘coherent perfect absorption’ (CPA). The CPA concept is related to that of ‘critical coupling,’ well-known in microwave engineering, wherein light coupled to a cavity is strongly absorbed on resonance. CPA is now envisioned to provide the means for a host of novel optical switching phenomena. It has been proposed as a means for strong coupling to two-dimensional materials, and has been extended to microwaves and acoustics. To date, however, CPA has been realized in silicon, plasmonic systems, and metamaterial devices only over narrow bandwidths (typically a single wavelength or a few nanometers) using two coherent laser beams having a fixed phase relationship. While proposals have been made to produce CPA at two distinct wavelengths or over a broad bandwidth, experimental observations have not been forthcoming.
In view of the state of, and the shortcomings, of the art, the inventors have recognized the benefits and advantages of the ability to controllably enhance the effective absorption of a specific material beyond its intrinsic absorption over a broad spectrum without modifying the material itself, using incoherent radiation, based solely on the judicious design of the photonic environment in which a layer of a lossy material is embedded. It would be particularly advantageous if complete absorption could be realized at all the resonances across an extended bandwidth range (one or more octaves) by all linear, lossy, planar photonic structures regardless of the details of their construction. The embodied apparatus, methods, and applications described in detail below and as recited in the appended claims enable the realization of such benefits and advantages.